The invention relates to a positioning device for the access arm of the magnetic head of a magnetic disk storage, which is positionable onto selective track addresses of the magnetic disk, with control of the magnetic head position for the track following by means of servo markings which are arranged at the magnetic disk in servo sectors of the recording tracks, staggered relative to each other in track direction and covering twice the width of a recording track, and with a switching device which is synchronized with the disk rotation, and which upon the sensing of the servo sectors by the magnetic head applies the sensed servo signals in predetermined intervals to a servo circuit.
It is known (GE-OS 2,404,309; U.K. Patent Specification No. 1,396,834) to design a positioning device for the access arm of the magnetic head of a magnetic disk storage in such a manner that the access arm, upon positioning of the magnetic head to a track address, is driven by a motor the speed of which is controlled during the positioning movement of the magnetic head by a servo device. This device effects an initial speed, a following uniform speed, and a delay of the access arm at the end of the positioning path before the magnetic head reaches the track address. The speed control of the access arm is effected by a servo circuit which upon reaching of the track address by the magnetic head, is switched off by the driving motor of the access arm. At the same time, the motor of the access arm is switched into a servo control circuit which initiates the guiding of the magnetic head over the center of a recording track.
In the above known device, servo and data markings are associated to different disk surfaces of a magnetic disk storage. The servo markings of a disk surface are sensed by a magnetic servo head, and the data markings of a disk surface are sensed by a magnetic data head. Both magnetic heads are associated jointly to an access arm which by means of an access motor positions the magnetic heads to a selected track addresses. When the track address has been reached, the servo control for the track following the magnetic heads is switched in which after a few oscillations are guided through the track following of the servo control over the center of the recoding tracks.
It is known, (GE-OS 2,202,747; U.S. Pat. No. 3,691,543) to form the magnetizing pattern of a servo track out of two adjacent track areas which in parallel spots, show positive-going transitions of the magnetic flux. In the intervals between these positive-going transitions of the magnetic flux each track area contains a negative-going transition of the magnetic flux which to the negative-going transitions of the magnetic flux of the adjacent tracks is arranged in staggered distance in track direction. From the positive-going transition of the magnetic flux the clock signals are derived, and the negative-going transitions of the magnetic flux arranged in spaced formation supply the servo signals for the track centering of the magnetic heads. The device contains an automatic amplifier control by means of which upon the sensing of the servo signals a correction of the signal amplitudes is achieved when the magnetic servo head reaches differing flight heights in different track areas of the magnetic disk. The amplification control consists in that the signal amplitudes are added up by two negative-going transitions of the magnetic flux. With the same flight height of the magnetic head the sum of the two signals is constant, independently of the deviation from the track center. A changing of the sum signal is determined by comparing it with a reference signal. The difference of the sum signal from the reference signal is evaluated for the amplification control which considers the changing of the flight height of the magnetic head. The servo circuit can thus compensate signal errors originating from the changing of the flight height of the magnetic servo head.
Although by means of the known device it is possible to obtain an automatic amplification control for considering the flight height of the magnetic servo head, it is necessary to generate in each clock period of the system at least two servo signals having different phase shifts. This results in a high transition frequency and a great band width of the magnetizing pattern, with the consequence of a low signal-to-noise ratio in the generation of the servo signals.
It is also known (GE-AS 1,424,516; U.S. Pat. No. 3,185,972) to control the track following of a magnetic head by means of servo signals which are sensed at servo markings that are recorded in servo sectors of the recording tracks. The servo sectors are arranged in the same angular distances between the data sectors of the magnetic disk. The sensing of the data and servo sectors of a magnetic disk is performed by one single magnetic head carrier which is positioned to the track address of a recording track. For that purpose, the servo circuit is controlled by a switching device synchronized with the disk rotation, and which upon sensing of the servo sectors by means of the magnetic head applies the sensed servo signals in predetermined intervals to the servo circuit. At the magnetic head carrier a servo magnetic head and a data magnetic head are arranged in parallel in a spacing of three recording tracks. Each address position of the magnetic head carrier has associated thereto two concentric recording tracks of the magnetic disk storage whose data sectors are sensable on the one hand by the magnetic servo head and on the other by the data magnetic head. The return of the magnetic servo head to the center of the servo track is still possible when it has been deflected by the 0.75-fold of an address spacing from the track center to the left or the right, respectively. The control range of the known device thus comprises a total of the 1.5-fold of the track spacing between two track addresses that are arranged concentrically one beside the other.
The disadvantage of the known device consists in that the density of the addressable recording tracks of a magnetic disk storage is limited. In the known device, there is furthermore the disadvantage than an automatic amplification control of amplitude controls of the servo signals caused by changes of the flight height of the magnetic servo head can be achieved only by a more implicate device.